In motor vehicles, and particularly trucks, motor homes and other heavy vehicles, there is a need for some means to retard the speed of the rear wheels, particularly on downgrades, to avoid "jackknifing" without requiring constant brake pressure. Excessive drag on the brake shoes may generate excessive heat and cause brake damage through friction. Others have sought to ease the problem by governing engine speed or applying braking means dependent upon engine speed but it is highly desirable to direct the control to, and limit the speed of, the rear wheels themselves to avoid a tendency of same to overrun the others.